So first things first: Flintlock Fantasy is a subgenre of Fantasy (actually it’s a subgenre of a subgenre) that is kind of unofficial. It’s not widely known, nor used as a common tag, but it’s acknowledged if small. The genre name comes from the use of flintlock weapons, a type of mechanism in guns. It’s usually applied to novels within the fantasy genre that incorporate flintlock weapons and other similar technology.
This genre doesn’t appear to be widely known/acknowledged, largely because works that would overlap with the subgenre are also often within other subgenres that are better known and easier to market. It also appears to be a bit subjective what the “fantasy” elements need to be. For example, one could argue that any pirate adventure novels that take on any element of fantasy are flintlock fantasy, due to the popular use of flintlock weapons in pirate literature. But the presence of legends about mermaids doesn’t necessarily lend itself to a fantasy description.
Within the genre of flintlock fantasy, it seems the most important elements are outright magic and the presence of flintlock weaponry, as well as other period appropriate technology. Essentially, the subgenre’s goal is just to present another timeframe based in our reality in which to have fantasy stories without setting them in our actual real world.
I think that long story short, flintlock fantasy as a subgenre is just another tag to add to existing fantasy genres. Perhaps as time goes on and it becomes more widely known and marketable, we’ll see more authors promoting their work under the flintlock fantasy descriptor. Until then, I think it’ll largely be used by people like me, book bloggers and readers trying to find a new way to categorize existing books for list and readathon purposes.